1. Field of the Invention
The present invention relates first and foremost to a method of transmitting data between a source of data to be transmitted, connected to the input of a transmission channel, and a circuit for using the data received from said channel.
Such a method is used, for example, for transmitting data between a buoy detecting underwater targets, such as submarines, and an aircraft, e.g. an airplane, charged with using the measurements made by this buoy.
2. Description of the Prior Art
Such methods are known used in particular for acoustic type buoys, i.e. which detect the acoustic waves transmitted by a target. In this case, the analog signal at the output of the acoustic sensor provided on the buoy modulates the frequency of a carrier, which is transmitted by short wave link to the aircraft. On board the latter, the carrier is demodulated so as to recover the useful signal, which is then transmitted to the user circuit.
Such a method is however not usable for magnetometric type buoys, i.e. which detect the modifications of the Earth's magnetic field related to the presence of a target. In this case, and as is known, the buoy is provided with a very sensitive magnetometer, e.g. of nuclear magnetic resonance type, which measures at regular time intervals the value of the magnetic field in which the buoy is situated. These values are generally available in digital form, each of them being represented by a 24 bit word, for example. The succession of digital words must then be transmitted to the aircraft, where it is processed in the user circuit. When the succession of bits representing the succession of digital words to be transmitted is coded directly, for example, by means of a bi-phase code, and when the carrier is modulated by means of the coded signal, the signal received by the aircraft is not usable, because of the excessive degradation of the data which occurs during passage through the transmission channel, comprising the transmission antenna, the short wave link and the reception antenna. In fact, in the presence of waves, it may happen that the two antennae are no longer in direct sight of each other, for example because the transmission antenna is masked by the waves. Such masking may sometimes last for several seconds, and the result in practice is an interruption, of the same duration, in the transmission between the buoy and the aircraft. This phenomenon, not very troublesome in the case of acoustic type buoys, considering the nature of the data transmitted, may, on the contrary, be very troublesome in the case of magnetometric type buoys. In fact, in the case of a magnetometric type buoy, the processing in the user circuit is carried out on very long signal "time slices". consequently, an error over several consecutive data words disturbs the use of the signal over a much longer period, which could not be tolerated. Calculation shows then that, under nominal connection conditions for which the rate of transmission of the words is usually of the order of about 10 per second, the probability of having a false word must be of the order of 10.sup.-6 to 10.sup.-7.
Moreover, a method for transmitting digital data is known which tolerates interruptions in the transmission, and which is usually designated by interleaving method. Such a method is described for example in the work "Error correction for digital communication" by CLARK-CAIN, Plenum Press. In this method, tables are formed each containing a plurality of words to be transmitted, each of the words to be transmitted is coded by means of an error correcting code, and the tables thus formed and coded are stored temporarily. Then, for transmitting the bits of a table to the input of the transmission channel, the first bit of the first word is transmitted but, instead of continuing with the second bit of the first word, the third bit of the first word and so on, after the first bit of the first word, the first bit of the second word is transmitted, the first bit of the third word, and so on. In this case, when an interruption in the transmission occurs, instead of affecting all the bits of the same word, and even several successive words, it affects for example all the n.sup.th bits of several successive words, which is without importance for, because of the error correcting code, the errors in the words received are corrected during decoding.
Such a method thus makes it possible, when it is associated for example with a code capable of detecting and correcting a false bit per word received, to overcome the problems caused by interruptions in the transmission of a duration equal to the product of the duration of a bit transmitted by the number of words in each table. However, this is done at the cost of introducing, between the data source and the user circuit, a delay which can be shown is substantially equal to twice the duration of formation of a table. In fact, before the value of a received word is available, it is necessary to wait until all the words of the table have been received in which it is situated, and obviously a certain time was necessary at transmission for forming this table. For example, when the number of bits per coded word to be transmitted is equal to 16, and when it is desired to be able to tolerate an interruption lasting two seconds, the delay introduced by the interleaving method is about 64 seconds, which is already considerable. In the case of a magnetometric buoy where the words to be transmitted comprise, for coding, a number of useful bits equal to 24, the number of bits per coded word becomes for example equal to 32 and the delay introduced is even greater, which means that the aircraft risks detecting the underwater target when the latter is already too far away.